A conventional optical recording/reproducing device, here a magneto-optical memory device, will be described hereinbelow. A magneto-optical disk device has been chosen as a concrete example for the magneto-optical memory device. This magneto-optical disk device uses a magneto-optical disk as optical recording medium and is capable of recording, reading and erasing information on the magneto-optical disk. The following description will be covered with reference to FIGS. 26 to 33.
First, operations for recording information on the magneto-optical disk and for erasing information recorded on the magneto-optical disk will be discussed with reference to FIG. 26.
As illustrated in FIG. 26(a), a typical magneto-optical disk is constituted of disk substrate 2804 and a recording magnetic film 2805 formed on the disk substrate 2804. The recording magnetic film 2805 is formed such that its axis of easy magnetization is perpendicular to the film surface thereof, and is initialized such that the direction of magnetization indicated by an arrow A or an arrow B within the film shown in FIG. 26(a) is preliminary set in a fixed direction (for example, shown by the arrow A in FIG. 26(a)).
During recording, a laser beam 2803 is projected from a semiconductor laser 2801, converged by an objective lens 2802 so as to have a diameter of approximately 1 .mu.m and is irradiated on the recording magnetic film 2805. At this time, the intensity of the laser beam 2803 is controlled according to a recording signal 2807 (see FIG. 26(b)) corresponding to the information to be recorded. When the recording signal 2807 is in the high level and thereby the intensity of the laser beam 2803 is strong, the temperature of the area illuminated by the strong laser beam 2803 rises locally, goes above the Curie point, and the coercive force of the area illuminated significantly lowers. As a result, the direction of magnetization A in the area where the coercive force lowered is inverted and frozen in the same direction of magnetization B as that of an external magnetic field 2806 that was applied preliminary, thereby permitting information corresponding to the recording signal 2807 to be recorded on the recording magnetic film 2805. Hereinafter, parts where high level recording signals 2807 were recorded as described above and where the direction of magnetization is B will be referred to as marks 2809, and parts where low level recording signals 2807 were recorded and where the direction of magnetization is A will be referred as non-marks 2810. In other words, the marks 2809 correspond to, for instance, the binary codes "1" composing the information, while the non-marks 2810 correspond to the binary codes "0". Hereinafter, the method of recording information as described above will be referred to as magneto-optical recording.
Information recorded on the recording magnetic film 2805 is erased by inverting the direction of the external magnetic field 2806 and following a method similar to the one used for recording. The direction of magnetization is restored to its original direction of initialization, i.e. the direction of magnetization A in FIG. 26(a), and the recorded information is erased. Marks 2809 thus become non-existent in the erased part.
In the present example, the light modulation method is adopted, i.e. recording is executed by modulating the intensity of the laser beam 2803 in accordance with the recording signal 2807, while applying an external magnetic field 2806 of a constant intensity. However, the magnetic modulation method may as well be adopted and recording can be executed by making the intensity of the laser beam 2803 constant and modulating the direction of the external magnetic field 2806 in accordance with the recording signal 2807.
The disk substrate 2804 mentioned earlier is made of glass, plastic or other material, and lands and pits 2808 are preliminary etched thereon, as shown in FIG. 26(a). The lands and pits 2808 represent address information indicating the addresses of tracks and sectors. The above address information is preliminary etched onto the disk substrate 2804 during the manufacturing stage of the magneto-optical disk according to a fixed format. Hence, the lands and pits 2808 cannot be recorded or erased thereafter. Hereinafter, parts where a plurality of lands and pits 2808 are formed in a group will be referred to as pre-formatted sections 3003. Information is recorded and erased in areas other than the pre-formatted sections 3003. Hereinafter these areas will be referred to as MO (magneto-optical) sections 3002. Pre-formatted sections 3003 and MO sections 3002 are usually accommodated alternately to form a track 3005 in a spiral shape or in the shape of concentric circles, as illustrated in FIG. 28. A sector 3004 is constituted by a pair composed of a pre-formatted section 3003 and a MO section 3002. In addition, a magneto-optical disk 3001 comprises a plurality of sectors 3004 formed on the track 3005, each sector 3004 being provided with address information. Information is recorded, reproduced and erased sector 3004 by sector 3004.
As illustrated in FIG. 29, the pre-formatted sections 3003 of the track 3005 are arranged such that either the land or the pit that compose one land and pit 2808 shown in FIG. 26(a) forms a mark 2811, and such that the other component of the land and pit 2808 forms a non-mark 2812. Marks 2809 and non-marks 2810 are recorded in the MO section 3002 through magneto-optical recording as described earlier.
A reproduction operation performed on the magneto-optical disk 3001 will be discussed hereinbelow with reference to FIG. 27.
As illustrated in FIG. 27(a), the laser beam 2803 is projected from the semiconductor laser 2801, is converged by the objective lens 2802 so as to have a diameter of approximately 1 .mu.m and is irradiated upon the recording magnetic film 2805. Here, the intensity of the laser beam 2803 is weaker when information is reproduced than when information is recorded or erased. The laser beam 2803 is a linearly polarized light and its plane of polarization is rotated as the laser beam 2803 passes through or is reflected by the recording magnetic film 2805 due to the Faraday effect or the Kerr effect. The plane of polarization of the laser beam 2803 is rotated in mutually opposite directions depending on whether the laser beam 2803 is irradiated on a mark 2809 or a non-mark 2810. Reproduction of recorded information is performed by detecting the difference in polarization direction. Accordingly, two types of reproduced signals S1 and S2, shown by (b) and (c) in FIG. 27, are generated.
The reproduction optical system employed for producing the reproduced signals S1 and S2 will be discussed briefly hereinbelow. As illustrated in FIG. 30, a reflected light 3201 coming from the magneto-optical disk 3001 is directed toward a PBS (analyzer) 3202 where it is split according to its polarization direction through the Kerr effect. Two detected lights 3210 and 3211 that were separated in the PBS 3202 are respectively directed toward photodetectors 3203 and 3204 where they are converted into electric signals that vary according to the respective intensities of the detected lights 3210 and 3211, and released as reproduced signals S1 and S2. As it will be covered in details later, the signals corresponding the pre-formatted section 3003 and the signals corresponding to the MO section 3002 can be obtained separately by determining the sum and the difference of the reproduced signals S1 and S2. In addition, the marks 2809 and the non-marks 2810 may be reproduced separately through the signals corresponding to the MO section 3002 thereby enabling the information recorded on the recording magnetic film 2805 to be reproduced.
The polarity of the reproduced signals S1 and S2 will be described with reference to FIG. 31.
Suppose that .alpha. represents the vector of a reflected light from a non-mark 2810 (direction of magnetization A) of the MO section 3002, and .beta. represents the vector of a reflected light from a mark 2809 (direction of magnetization B) of the MO section 3002. The reflected light vectors .alpha. and .beta. are rotated in opposite directions by an angle corresponding to the rotation angle of their respective plane of polarization. The X direction components and Y direction components of the reflected light vectors .alpha. and .beta. are detected in the PBS 3202 that transmits light having a X or Y polarization direction. These two polarization directions X and Y form a right angle. Geometrical explanation will be made hereinbelow. The reflected light vector .alpha. is projected in the polarization direction X and the polarization direction Y thereby producing detected light vectors .alpha..sub.x and .alpha..sub.y. Similarly, the reflected light vector .beta. is projected in the polarization direction X and the polarization direction Y thereby producing detected light vectors .beta..sub.x and .beta..sub.y. The magnitudes of detected light vectors .alpha..sub.x and .beta..sub.x correspond to the reproduced signal S1 and the magnitudes of the detected light vectors .alpha..sub.y and .beta..sub.y corresponds to the reproduced signal S2. Further, the detected light vectors .alpha..sub.x and .beta..sub.x correspond to the detected light 3210 shown in FIG. 30, and the detected light vectors .alpha..sub.y and .beta..sub.y correspond to the detected light 3211.
Suppose that, as illustrated in FIG. 31, the high level of the reproduced signal S1 corresponds to a non-mark 2810 and the low level of the reproduced signal S1 corresponds to a mark 2809. Here, the high level of the reproduced signal S2 corresponds to a mark 2809 and its low level to a non-mark 2810. The polarity of the reproduced signal S1 and the polarity of the reproduced signal S2 are thus opposite. The reproduced signals S1 and S2 are then fed into a differential amplifier where the difference of the reproduced signals S1 and S2 is determined and the reproduced signals S1 and S2 are amplified and thereby their S/N is improved, and information is reproduced.
The polarity of the reproduced signals S1 and S2 obtained when the pits and lands 2808 physically etched in the pre-formatted sections 3003 are reproduced, will be described hereinbelow with reference to FIG. 32. As there is no recording nor erasing operation taking place in the pre-formatted sections 3003, the direction of magnetization therein coincides with the direction A only. When the laser beam 2803 is irradiated on a pre-formatted section 3003, the shape of the marks 2811 and non-marks 2812, i.e. the lands and pits 2808, causes the laser beam 2803 to be diffracted. As a result, a long reflected light vector .delta. (when, for example, a non-mark 2812 is read) or a short reflected light vector .epsilon. (when, for example, a mark 2811 is read) is produced in accordance with the lands and pits 2808, as illustrated in FIG. 32. A detected light vector .delta..sub.x and a detected light vector .delta..sub.y are produced by projecting the reflected light vector .delta. in the polarization direction X and in the polarization direction Y of the PBS 3202. Similarly, a detected light vector .epsilon..sub.x and a detected light vector .epsilon..sub.y are produced by project the reflected light vector .epsilon. in the polarization direction X and in the polarization direction Y of the PBS 3202. The magnitudes of the detected light vector .delta..sub.x and of the detected light vector .epsilon..sub.x correspond to the reproduced signal S1, and the magnitudes of the detected light vector .delta..sub.y and of the detected light vector .epsilon..sub.y correspond to the reproduced signal S2. The high level of the reproduced signal S1 and the high level of the reproduced signal S2 both correspond to a non-mark 2812 of the lands and pits 2808, while the low level of the reproduced signal S1 and the low level of reproduced signal S2 correspond to a mark 2811. Consequently, as illustrated in FIGS. 27(b) and (c), the reproduced signals S1 and S2 have the same polarity for the pre-formatted section 3003 while they have mutually inverted polarities for the MO section 3002.
The reproduction circuit of the magneto-optical disk device will be described with reference to FIG. 33.
In FIG. 33, the reproduced signals S1 and S2 are fed into a reproduction circuit 3501 where a binary output signal 3510 is derived from the reproduced signals S1 and S2. The output signal 3510 is sent to an address generating circuit 3502 and to a timing generating circuit 3503. In the address generating circuit 3502, the address information contained in the pre-formatted section 3003 of each sector 3004 shown in FIG. 28, is read from the output signal 3510 enabling an address signal 3511 to be generated and released. In the timing generating circuit 3503, a sector mark used for the synchronization of the sectors and also contained in the pre-formatted section 3003, is detected enabling a recording/reproducing/erasing reference timing signal 3512 to be generated and released. Provision is made such that the magneto-optical disk device records, reads or erases information in the sector 3004 of the desired address based on the address signal 3511 and the recording/reproducing/erasing reference timing signal 3512.
However, the arrangement conventionally adopted suffers from the following drawbacks. Namely, when the magneto-optical disk whereon information was recorded through magneto-optical recording is to be used with a different device, or when magneto-optical disks having different characteristics (for example different reflectances, different transmittances, etc.) are used, the recording/reproducing conditions vary, sometimes causing the reproduction to be infeasible. In addition, when information is being read from a magneto-optical disk, the projection of a laser beam having an inadequate light intensity might erase the information recorded on the magneto-optical disk, or even lead to the destruction of the magneto-optical disk itself. An optical recording/reproducing device having a higher reliability was thus necessary.